1. Field of the Invention
This invention relates to a process for producing a vinyl chloride polymer.
2. Description of the Prior Art
Vinyl chloride polymers are commonly produced through the following steps. A vinyl chloride monomer alone or a mixture of a vinyl chloride monomer and a vinyl monomer copolymerizable therewith, deionized water, a polymerization initiator and a dispersant are introduced into a polymerization vessel. Maintaining these at a stated temperature (usually from 30 to 75xc2x0 C.) with stirring, polymerization is carried out until it comes to a conversion of 60 to 98%. At a point of time where the conversion has reached a stated value, a polymerization inhibitor is introduced to stop the polymerization. After the polymerization has been completed, an unreacted monomer or monomers remain(s) in a large quantity in the polymerization vessel and in the resultant vinyl chloride polymer slurry. Accordingly, after the polymerization has been stopped, the unreacted monomer is recovered out of the polymerization vessel by evacuation. Thereafter, the polymer slurry obtained by the polymerization reaction is withdrawn out of the polymerization vessel into a blow-down tank. The polymer slurry is further fed into a monomer stripping column (hereinafter xe2x80x9cstripperxe2x80x9d), where the polymer slurry is subjected to stripping to remove the unreacted monomer remaining in the slurry. Then, the polymer slurry from which the residual unreacted monomer has been reduced is fed into a decanter, followed by dehydration, and thereafter the dehydrate is fed to a drying step to effect drying. Through the foregoing steps, a dried vinyl chloride polymer product is manufactured.
In recent years, in the manufacture of vinyl chloride polymers, it has become common to lower the weight ratio of water/monomer per one batch of polymerization to 1.50 or below to charge the monomer in a larger quantity in order to improve productivity. However, polymerization carried out at the water/monomer weight ratio lowered to 1.50 or below makes the unreacted monomer remain in the resultant polymer slurry in a larger quantity.
Meanwhile, where the residual unreacted monomer is in a large quantity in the polymer slurry fed to the step of dehydration, almost all the monomer harmful to human beings is discharged to the atmosphere in the step of dehydration and the subsequent step of drying. Also, the residual monomer may remain in the dried vinyl chloride polymer product in a larger quantity. Thus, this is very unfavorable from the standpoint of safety and sanitation and environmental problems. Accordingly, the residual unreacted monomer in the polymer slurry obtained in the step of polymerization must sufficiently be reduced by means of the stripper. As a method by which the residual unreacted monomer in the polymer slurry is more reduced, a method of feeding steam to the stripper in a large quantity is known in the art. This method, however, is undesirable from the viewpoint of energy saving and cost reduction, and also may cause burnt matter of vinyl chloride polymers in the stripper because of the steam. Such burnt matter may remain in final products as foreign matter to cause a problem. Accordingly, it has been sought to provide a method by which the residual unreacted monomer in the polymer slurry can efficiently be removed using steam in a smaller quantity in the stripper.
An object of the present invention is to provide a process for producing a vinyl chloride polymer at a high productivity, in the stripping step of which the residual unreacted monomer in the polymer slurry can efficiently be removed using steam in a smaller quantity.
As a result of extensive researches and studies on the above problems the prior art has had, the present inventors have discovered that, when the proportion of water/monomer is lowered to a weight ratio of 1.50 or below to charge the monomer in a larger quantity, the polymer slurry may be fed into the stripper after the viscosity (20xc2x0 C.) of the resultant vinyl chloride polymer slurry has been adjusted to a specific range by further adding water to carry out stripping, whereby the residual unreacted monomer in the polymer slurry can effectively be reduced to bring about an improvement in productivity per one batch of polymerization. Thus, they have accomplished the present invention.
More specifically, the present invention provides a process for producing a vinyl chloride polymer, comprising the steps of:
(a) subjecting a vinyl chloride monomer alone or a mixture of a vinyl chloride monomer and a vinyl monomer copolymerizable therewith, to suspension polymerization in water in the presence of a polymerization initiator and a dispersant;
(b) subjecting the resultant vinyl chloride polymer slurry (hereinafter often xe2x80x9cpolymer slurry Axe2x80x9d) to stripping to remove an unreacted monomer remaining in the slurry; and
(c) dehydrating the polymer slurry having been subjected to stripping;
wherein, in the polymerization step (a), the proportion of water/monomer is set in a weight ratio of from 0.80 to 1.50, and the viscosity at 20xc2x0 C. of the polymer slurry A to be fed to the stripping step is previously kept adjusted to 0.30 Paxc2x7s (300 cP) or lower.
The present invention will be described below in detail.
The polymerization step is the step of subjecting a vinyl chloride monomer alone or a mixture of a vinyl chloride monomer and a vinyl monomer copolymerizable therewith (hereinafter often xe2x80x9cmonomer mixturexe2x80x9d), to suspension polymerization in water in the presence of a polymerization initiator and a dispersant to obtain a vinyl chloride polymer slurry. In the process of the present invention, it is essential that the proportion of water/monomer is set in a weight ratio ranging from 0.80 to 1.50.
This polymerization step is described below in greater detail.
A vinyl chloride monomer or monomer mixture, deionized water, a polymerization initiator and a dispersant are introduced into a polymerization vessel, and polymerization is carried out maintaining a stated temperature (from 30 to 75xc2x0 C.) with stirring. At a point of time where the conversion has reached a stated value (60 to 98%), a polymerization inhibitor is introduced to stop the polymerization.
As the monomer, a vinyl chloride monomer or a monomer mixture of a vinyl chloride monomer and a vinyl monomer copolymerizable with the vinyl chloride monomer (usually containing 50% by weight, and preferably 70% by weight or more, of the vinyl chloride monomer) is used. The copolymerizable vinyl monomer in the monomer mixture may include monomers exemplified by xcex1-olefins such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene, 1-tridecene and 1-tetradecene, vinyl esters such as maleic acid, maleic anhydride, vinyl acetate and vinyl propionate, vinyl ethers such as lauryl vinyl ether and isobutyl vinyl ether, vinylidene chloride, acrylic acid, styrene, xcex1-methylstyrene, and acrylonitrile. Any of these may be used alone or in combination of two or more types.
The polymerization initiator may include those exemplified by diacyl organoperoxides such as isobutyryl peroxide, 3,5,5-trimethylhexanolyl peroxide and lauroyl peroxide, peroxyester organoperoxides such as cumyl peroxyneodecanoate, t-butyl peroxyneoheptanoate, t-butyl peroxyneodecanoate and t-hexyl peroxyneodecanoate, and peroxydicarbonate organoperoxides such as diallyl peroxydicarbonate and bis(2-ethylhexyl) peroxydicarbonate. Any of these may be used alone or in combination of two or more types. Any of these organoperoxide type polymerization initiators may usually be used in an amount of from 0.01 to 0.3 part by weight, and preferably from 0.03 to 0.2 part by weight, based on 100 parts by weight of the whole monomer to be charged.
The dispersant may include those exemplified by cellulose derivatives such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose and hydroxypropyl methyl cellulose, water-soluble or oil-soluble partially saponified polyvinyl alcohols, and water-soluble polymers such as acrylic acid polymer and gelatin. Any of these may be used alone or in combination of two or more types. Also, any of these dispersants may be used in combination with at least one of nonionic emulsifiers such as sorbitan monolaurate, sorbitan trilaurate, glycerol tristearate and an ethylene oxide-propylene oxide block copolymer, and anionic emulsifiers such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glycerol oleate and sodium lauryl sulfate. The dispersant may usually be used in an amount of from 0.02 to 0.2 part by weight, and preferably from 0.05 to 0.15 part by weight, based on 100 parts by weight of the whole monomer to be charged.
In the present invention, the proportion of the deionized water to monomer to be charged, i.e., the ratio of water/monomer (weight ratio) is usually from 0.8 to 1.5, and preferably from 1.0 to 1.3. If this water/monomer ratio is more than 1.5, the production (productivity) per one batch of polymerization may lower. If on the other hand the water/monomer ratio is less than 0.8, the viscosity of the slurry may increase with progress of polymerization reaction and the stirring may become imperfect to cause coarse particles, which can be the cause of fish eyes. Also, the slurry may have low heat transfer properties to make heat not well removable through the jacket, bringing internal-temperature control into bad condition to cause difficulties such that the reaction runs away in some cases.
The monomer is consumed with progress of polymerization reaction such that the pressure is not more than the saturated vapor pressure of monomer in the gaseous phase portion inside the polymerization vessel, whereupon the internal pressure of the polymerization vessel begins to lower. After this internal pressure begins to lower, it becomes able to catch conversion. Accordingly, the conversion is judged from the internal pressure of the polymerization vessel, and the polymerization inhibitor is introduced at a point of time where it has come to a stated polymerization vessel internal pressure (conversion), to stop polymerization. The point of time to stop polymerization is at the time the conversion has reached 60 to 98%. In order to more improve productivity, it may preferably be at the time the conversion has reached 80 to 95%.
The polymerization inhibitor may include those exemplified by ethylenebis(oxyethylene)bis[3-(5-t-butyl-4-hydroxy-m-tolyl)propio nate], 2,6-di-t-butyl-p-t-butyl-4-hydroxyanisol (2-BHA), 2,2xe2x80x2-methylenebis(4-methyl-6-t-butylphenol (MBMBP), polybutyrated bisphenol A, and bisphenol A. Any of these may be used alone or in combination of two or more types.
The polymerization inhibitor may usually be used in an amount of from 0.0001 to 0.5 part by weight, and preferably from 0.001 to 0.05 part by weight, based on 100 parts by weight of the whole monomer having been charged.
After the polymerization has been stopped, the unreacted monomer is recovered out of the polymerization vessel by evacuation, and then the vinyl chloride polymer slurry obtained is withdrawn out of the polymerization vessel into a blow-down tank.
The stripping step is the step of subjecting the vinyl chloride polymer slurry obtained in the polymerization step, to stripping to remove the unreacted monomer remaining in the slurry. In the process of the present invention, it is essential that the viscosity at 20xc2x0 C. of the polymer slurry to be fed into the stripper is previously kept adjusted to 0.3 Paxc2x7s (300 cP) or lower.
This stripping step is described below in greater detail.
After the polymerization has been stopped, it is usually intended to remove the unreacted monomer by evacuation. Even after this operation, however, usually from about 3,000 to 30,000 ppm of unreacted monomers remain in the polymer slurry without being completely recovered. Accordingly, the polymer slurry in the blow-down tank is fed into a stripper, where the polymer slurry is subjected to stripping to remove the unreacted monomer remaining in the slurry. The removal of the residual unreacted monomer by stripping may be made by any method commonly used. For example, a method disclosed in Japanese Laid-open Publication (Kokai) No. 54-8693 is available. This method is a method in which, into a stripper having in the column a plurality of trays made of perforated plates, the polymer slurry containing the residual unreacted monomer is fed from its column top and steam is fed from its column bottom. In this method, the polymer slurry falls through the plurality of trays made of perforated plates in the column, in the course of which it comes into contact with the steam coming up from the column bottom through the pores of the trays, so that the unreacted monomer remaining in the polymer slurry vaporizes to become separated. Thus, the residual unreacted monomer is recovered from the column top and the polymer slurry is withdrawn out from the column bottom. Through this step, the content of the unreacted monomer remaining in the polymer slurry becomes not more than 50 ppm, and preferably not more than 10 ppm.
The viscosity of the polymer slurry to be fed into the stripper may be adjusted by a method described below.
In the present invention, the proportion of the water and monomer charged for polymerization, the water/monomer ratio, is set as low as 0.80 to 1.50 in order to improve the productivity per one batch of polymerization. Hence, the polymer slurry obtained after the polymerization is completed has usually a viscosity of from 0.5 to 1.0 Paxc2x7s ((500 to 1,000 cP), and may have a viscosity higher than 1.0 Paxc2x7s in some cases. In the case of such a polymer slurry having a viscosity of about 0.5 to 1.0 Paxc2x7s, the unreacted monomer remaining in the polymer slurry appears to insufficiently come in contact with the steam when fed into the stripper. In the process of the present invention, it is essential that the viscosity at 20xc2x0 C. of the polymer slurry to be fed into the stripper is adjusted to 0.3 Paxc2x7s (300 cP) or lower, and preferably 0.1 Paxc2x7s (100 cP) or lower. If the polymer slurry to be fed into the stripper has a viscosity higher than 0.3 Paxc2x7s, the unreacted monomer remaining in the polymer slurry can not efficiently be removed even by stripping and may remain in a large quantity, so that the residual unreacted monomer is in a large quantity in the polymer slurry fed to the step of dehydration, and almost all the monomer harmful to human beings is discharged to the atmosphere in the step of dehydration and the subsequent step of drying. Also, the residual monomer may remain in the dried vinyl chloride polymer product in a larger quantity. Thus, this is very unfavorable from the standpoint of safety and sanitation and environmental problems.
In the process of the present invention, the viscosity of the polymer slurry to be fed into the stripper is adjusted by adding water to the polymer slurry obtained after the polymerization is completed. There are no particular limitations on the water to be added. Deionized water may be used like the water charged for polymerization.
With regard to the time and place (place in the apparatus) at which this water is added, there are no particular limitations thereon as long as it is added before the polymer slurry is fed into the stripper. When, however, the water is added in the step of polymerization, it is so added that the water/monomer ratio is kept within the range of from 0.80 to 1.50 so as not to lower the productivity per one batch of polymerization.